1. Field of the Invention
This invention relates to a focus condition detecting device in an image forming optical system.
2. Description of the Prior Art
The focus condition detecting device in the conventional image forming optical system has consumed much electric power because the photoelectric device or the image forming optical system is driven in the direction of the optic axis and has generated vibration and noise while encountering difficulties in making the device compact. To overcome these disadvantages, focus condition detecting devices in which a light image is moved on the light-receiving surface of a photoelectric device in accordance with the image forming position of the image forming optical system so that the focus condition is detected by the detection of this movement have been proposed in Japanese Open patent application Publication No. 15432/1976.
The technique disclosed in the aforementioned Publication will hereinafter be described by reference to FIG. 7 of the accompanying drawings.
In an image forming optical system comprising an image forming lens 1 and a fixed focal plane 30, small mirrors 32a and 32b functioning as a pair of light beam separating elements are disposed at positions spaced apart from the optic axis 31 just behind the lens 1. A photoelectric converting element array 7a comprising a number of photoelectric converting elements N.sub.1, N.sub.2, N.sub.3, . . . , N.sub.n each having a small light-receiving surface such as a photoconductor and a photoelectric converting element array 7b comprising similar photoelectric converting elements N'.sub.1, N'.sub.2, N'.sub.3, . . . , N'.sub.n are disposed at two positions of the reflecting optical system formed by the mirrors 32a, 32b which are conjugate with the fixed focal plane 30.
The light rays from an object 33 on the optic axis 31 form a light image 33' focused on the fixed focal plane 30. At this time, identical images are formed on the positionally corresponding elements N.sub.1 .multidot.N.sub.1 ', N.sub.2 .multidot.N.sub.2 ', . . . , N.sub.n .multidot.N.sub.n ' of the pair of photoelectric converting element arrays. For example, the light from the end 33a of the object 33 is imaged on the elements N.sub.3 and N'.sub.3, respectively, of the two element arrays. An object 34 indicated by a dash line lies at a position on the optic axis which is spaced further apart from the lens 1 than the object 33 and the light image 34' thereof is the front focus imaged at a location nearer to the image forming lens 1 than the fixed focal plane 30, namely, at a location before the fixed focal plane 30. At this time, the light image formed in an out-of-focus condition on the upper element array 7a is displaced leftwardly (toward N.sub.1) from the light image 33' during the in-focus condition. The light image formed in an out-of-focus condition on the lower element array 7b is displaced leftwardly (toward N.sub.n ') from the light image 33'.
In FIG. 7, for the sake of simplicity, only the light ray from a portion 34b of the object 34 which is on the optic axis 31 is indicated by dash line.
Although not shown, in the case of the rear focus imaged at a location further from the image-forming lens 1, conversely to the case of the front focus, the light image on the upper element array 7a is displaced rightwardly (toward N.sub.n) from the light image 33', the light image on the lower element array 7b is displaced rightwardly (toward N.sub.1 ') from the light image 33'. That is, during the out-of-focus condition, the light images on the pair of element arrays are positionally deviated from each other.
According to this technique, the out-of-focus and the in-focus condition can be discriminated from each other by applying a circuit for comparing and discriminating the mutually corresponding elements (for example, N.sub.1 versus N'.sub.1, N.sub.2 versus N.sub.2 ') of the pair of photoelectric converting element arrays.
However, said Publication does not disclose means for discriminating the front focus formed forwardly of the photoelectric converting element arrays and the rear focus formed rearwardly of the photoelectric converting element arrays.